Load cells have long been used to measure forces and loads. Such load cells generally utilize a resilient structure which is tensioned, compressed, or bent by a force applied to the structure. Strain-sensing devices, such as wire or semiconductor strain gauges, are mounted to measure the force-induced tension, compression, or bending.
In many applications, it is important that the load cell respond to some types of forces but be insensitive to other types of forces. For example, a load cell used to support a weighing tray should respond to objects placed on the tray, but it should be insensitive to the position of the objects on the tray. In other words, the load cell should respond to the downward force exerted on the load cell by the object, but it should be insensitive to moments generated by the weight of the object.
Load cells utilizing a cantilever beam have sometimes been used to weigh objects. In a cantilever beam load cell, a beam projects generally perpendicularly from a support, and a load is carried by the beam so that it deflects in proportion to the weight of the load. A strain-sensing device is mounted on the beam, usually on the upper and lower surfaces, to provide an electrical indication of the load. The principal disadvantage of cantilever beam load cells is the difficulty of coupling a load to the beam without also transmitting load-induced moments to the beam. This difficulty is usually overcome by either hanging the load from the cantilever beam or supporting the load on a knife edge mounted on the beam so that the load is incapable of transmitting moments to the beam.
Another technique for overcoming the difficulty encountered by loading cantilever beams is to provide a stabilizing structure which resists moments but is highly compliant to linear forces. The stablizing structure thus constrains a force-transmitting member to move in a single direction so that it can deflect the cantilever beam, but, since it is incapable of rotating, it cannot apply a moment to the beam.
One commonly used load cell utilizes a pair of strain gauge instrumented parallel beams. Basically, the parallelogram load cell includes a pair of parallel, spaced-apart load-receiving members which are interconnected by parallel beams. Moments applied to the force-receiving members are translated into forces applied to the beams along their longitudinal axes. Although the beams do, in fact, elongate and contract in response to these moments, strain-sensing devices mounted on the upper and lower surfaces of the beams are connected differentially so that the strain gauges respond to bending but not elongation and compression.
Parallelogram load cells of this type are quite similar to cantilever load cells, but, in fact, they are not in actuality cantilever beams. In a cantilever beam, the beam deflects in a single curve from a horizontal angle at the support to either an upward or downward angle at its end. The beams of conventional parallelogram load cells have ends with intersect their respective supports at ninety degrees, thus forcing the beam to bend in a compound S-shaped curved. A curvature of this type requires a relatively large number of strain-sensing devices and it is thus more expensive to manufacture.